Matter-matter Entanglement at a Distance

With the hybrid system thus generated, the researchers have realized a fundamental building block of a quantum network.

In the quantum mechanical phenomenon of “entanglement” two quantum systems are coupled in such a way that their properties become strictly correlated. This requires the particles to be in close contact. For many applications in a quantum network, however, it is necessary that entanglement is shared between two remote nodes (“stationary” quantum bits).

One way to achieve this is to use photons (“flying” quantum bits) for transporting the entanglement. This is somewhat analogous to classical telecommunication, were light is used to transmit informa-tion between computers or telephones. In the case of a quantum network, however, this task is much more difficult as entangled quantum states are extremely fragile and can only survive if the particles are well isolated from their environment.

The team of Professor Rempe of the Max Planck Institute of Quantum Optics has now taken this hurdle by preparing two atomic quantum systems located in two different laboratories in an entangled state: on the one hand a single rubidium atom trapped inside an optical resonator formed by two highly reflective mirrors, on the other hand an ensemble of hundreds of thousands of ultracold rubidium atoms which form a Bose Einstein condensate (BEC). In a BEC, all particles have the same quantum properties so that they all act as a single “superatom”.

In this experiment, the team of Professor Rempe has realized a building block for a quantum network consisting of two remote, entangled, stationary nodes. This is a milestone on the way to large-scale quantum networks in which, for example, quantum information can be transmit-ted absolutely safe.

In addition, such networks might help realizing a universal quantum computer in which quantum bits can be exchanged with photons between nodes designed for in-formation storage and processing.